In electron beam tubes of the traveling-wave type, a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic wave. In order to achieve the desired interaction, the electromagnetic wave is propagated along a slow-wave structure, such as an electrically conductive helix wound about the path of the electron stream. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure so that the traveling-wave may be made to effectively propagate at nearly the velocity of the electron stream. Slow-wave structures of the helix type are usually supported within an encasing barrel by means of a plurality of equally circumferentially spaced electrically insulating rods disposed between the helix and the barrel.
One method which has been employed to mount a helical slow-wave structure and its support rods within an encasing barrel, termed "triangulation", involves making the barrel in the form of a resilient tubular metallic clamp. The cross-section of the clamp is initially circular, with a cross-sectional area greater than that of the structure-rod subassembly to be inserted within the clamp, but with a normal diameter which is less than that of a circle circumscribing the structure-rod subassembly. The clamp is first distorted by applying forces to three points equally spaced along the circumference of the clamp to alter its cross-section from circular toward triangular and thereby produce a configuration which more closely conforms to that of the structure-rod subassembly. The structure-rod subassembly is then inserted into the distorted clamp with the rods intermediate the points of application of the forces. Upon removal of the distorting forces, the clamp restores itself toward its original shape and in so doing compresses the rods and the slow-wave structure into a rigid assembly. Further details regarding such triangulation techniques may be found in U.S. Pat. Nos. 2,943,228 to Bernard Kleinman and 3,514,843 to George Cernik.
Another technique which has been employed to mount a helical slow-wave structure and its support rods within an encasing barrel involves "heat-shrinking" the barrel. The barrel is heated to an elevated temperature, causing its inner circumference to expand. The subassembly of the helical slow-wave structure and its support rods are inserted into the barrel while the barrel is maintained at an elevated temperature. The barrel is then allowed to cool, causing it to shrink around the subassembly and to provide a tight interference fit with the support rods. For further details concerning such heat shrinking techniques, reference may be made to U.S. Pat. No. 3,540,119 to Arthur E. Manoly.
As traveling-wave tube operating frequencies have increased, required dimensions for slow-wave structures employed in such tubes have become smaller and smaller. Although the above-described methods of triangulation and heat shrinking are useful for securing larger-sized slow-wave structures in an encasing barrel, these methods have not been completely satisfactory for smaller-sized slow-wave structures, such as those operating at millimeter wavelengths.
A method which has been successfully employed to secure small-sized helical slow-wave structure-support rod subassemblies in an encasing barrel involves the "precision coining" of the subassembly. In this method a helical slow-wave structure is mounted between at least three support rods having a given circumscribing circular dimension. The slow-wave structure with its support rods are inserted into a tubular barrel having an interior diameter greater than the diameter of the circumscribing circular dimension. The barrel is malleably deformed so that its interior size is reduced to a diameter less than that of the circumscribing circular dimension to engage the support rods which in turn resiliently compress and deform the helical slowwave structure such that the compressed helical slowwave structure maintains a return force on the rods to firmly hold the helical slow-wave structure and support rods within the barrel. The precision coining technique is disclosed and claimed in co-pending application of George M. Lee, Ser. No. 789,882, filed Oct. 21, 1985, and assigned to the assignee of the present invention; now abandoned.